Dual Battery Systems: Starting & Auxiliary

About Dual Battery Systems

While the dual battery system isn’t quite necessary for a stationary off-grid setup; it is a near necessity when powering a Skoolie, or something similar, off grid. First, a typical “starting” battery for a car, or a Skoolie, is NOT the type of battery that you want to use to power your appliances while the engine is off. Without getting into the details, it’s bad for the battery and instead, we will be using “deep-cycle” batteries for our auxiliary battery bank. “Deep-cycle” batteries are specially engineered to withstand constant discharge and recharging cycles without depleting the batteries lifespan.

For most users, a simple isolating solenoid circuit will be sufficient; however, some people have different needs than others. I am going to show and explain the simple isolating solenoid that I am using in my truck to isolate and charge my auxiliary battery.

The solenoids operation is quite simple. It measures the voltages of the two batteries independent of one another. When the engine is running, the starting battery begins charging, like normal, nothing new here. The solenoid detects the high voltage of the alternator charging the battery, and it connects the starting battery to the auxiliary battery to allow the auxiliary battery to charge. When the engine stops and the voltages return to normal, the solenoid disengages the batteries. This leaves you free to use your auxiliary battery bank without fear of draining your starting battery.

HINT! The solenoid works in both directions. This means if you attached a solar panel and solar charger to your auxiliary battery, when the solar power charges the auxiliary battery, the solenoid will detect the increased voltage, connect the auxiliary battery with the starting battery, and thus charging the starting battery without running the engine. A dual battery system with solar charging is PERFECT for a Skoolie!!!

So lets get started with the dual battery system…

To do this right, you are going to want to use 1/0 or 0 gauge wire. It is somewhat more expensive than the cheaper option of 2/4/6 gauge wiring; however, 1/0 or 0 gauge is the safest wire you can use to transmit such high amperage without generating heat in the wire. No doubt, 2/4/6 gauge wire can work for shorter runs within the engine bay, but if you plan on housing your auxiliary battery bank more than 10 ft from the engine bay you should really consider 1/0 or 0 gauge wiring.

Wiring the Aux Battery

The first thing to consider was where I would mount the positive terminal fuse, as you can see in the 1st picture, I chose to mount the 100Amp fuse next to the positive terminal on the aux battery. I cut a roughly 10″ section of Red 1/0 Gauge Wire and hammer crimped terminal connectors to the ends of the wire. To finish each end, I wrapped each end with red electrical tape, leaving the terminal connectors exposed.

In this 2nd picture, you can see that I have cut a section of 1/0 gauge wire that is roughly 6′ long, and zip-tied it to the factory wiring along the engines firewall. Like the smaller 10” section, I hammer crimped terminal connectors to the ends of the 6′ wire, wrapped the ends with red electrical tape, and connected one side to the aux battery positive fuse block (seen in the 1st picture).

In this 3rd picture, you can see that I have taken the opposite side of the 6′ wire and connected it to one side of my battery isolator using a terminal connection. This concludes the wiring for the positive aux terminal.

This 4th picture shows how I grounded the negative side of the Aux battery to the chassis of my vehicle. The starting battery is grounded to the frame of your vehicle in the factory and therefor you can ground your aux battery by connecting its negative terminal to the chassis of the vehicle. If your setup does not allow for chassis grounding, you will need to use 1/0 gauge wire to connect the negative terminals of the starting and aux batteries.

Wiring the Aux Fuse Box

The Aux Fuse box is connected to the positive and negative terminals of the aux battery using 6awg wire. Like the larger 1/0 gauge wire, I hammer crimped terminal connectors to the ends of the wires, then wrapping the ends with electrical tape. For the positive wire, since I only had black 6awg wire, I wrapped the positive wire with red electrical tape for easy identification purposes. At the bottom of the 4th picture (above), you can see the 6awg wire wrapped in red electrical tape, it is connected to the positive end of the fuse box. The negative wire connects at the top of the Aux Fuse Block.

Wiring the Starting Battery

Fortunately, the Starting battery is already grounded due to the factory harness, this means, we simply need to connect the Starting battery positive terminal to the battery isolator, through a fuse. This picture shows the starting battery positive terminal connected to a 200A fuse, then connected from the fuse to the battery isolator terminal. We’re done with the Aux and Starting battery wiring!

Battery Monitor

The package that I ordered, T-MAX 47-3800 Dual Battery System, came with a Voltage Monitoring Panel that includes a button to manually link the batteries. The Panel needs 4 wires to be connected to the system, (Aux V+, Starting V+, GND, Link).